ZwCreateFile returns c000003a for a drive that does exist

Hello,
I have a Filter driver based on filemon for NT/ 2K/ XP. The problem is
that if i have only one mapped drive then that Drive hooking fails. But if
i have more than one mapped drive then the drive hooking fails for one
mapped drive and is successful for the rest. The error returned is
STATUS_OBJECT_PATH_NOT_FOUND. But the path does exist. It seems to be a
strange problem. Any clue would be appreciated.
Thanks.